Metabolic acidosis has a negative impact on prognosis of dialysis patients. The aim of this study was to determine the prevalence of severe metabolic acidosis in dialysis patients treated with sevelamer hydrochloride. In 2004, a nationwide survey (101516 dialysis patients) was conducted by the Japanese Society for Dialysis Therapy. We analyzed 32686 dialysis patients whose bicarbonate levels were measured in the survey. Sevelamer hydrochloride was prescribed to 9231 dialysis patients while 23455 dialysis patients were not prescribed sevelamer hydrochloride. In the present study, we defined severe acidosis as bicarbonate <15.8mmol/L. The mean serum bicarbonate level correlated significantly and negatively with the daily dose of sevelamer hydrochloride (R-2=0.806, P<0.0001). Logistic regression analysis indicated that the percentage of patients with severe acidosis increased significantly with increased dose of sevelamer hydrochloride (R-2=0.885, P<0.00001). The estimated doses of sevelamer hydrochloride associated with severe acidosis in 10% and 15% of patients were 3.5g/day (95% confidence interval [95%CI], 2.8-4.4) and 7.7g/day (95%CI=5.9-10.9), respectively. Severe acidosis was noted in 4.5% of patients who were not treated with sevelamer hydrochloride and in 16.1% of patients treated with sevelamer hydrochloride at 5.25g/day (P<0.0001). The results call for careful monitoring of serum bicarbonate level in hemodialysis patients treated with sevelamer hydrochloride.